The preparation of DMT is a well known process exemplified by the Witten process which utilizes p-xylene as a starting material. In the Witten process, the preparation of DMT can proceed via two pathways generally represented below as Routes 1 and 2. ##STR1##
In Route 1, p-xylene is oxidized [Ox], using methods known to one of ordinary skill in the art, to PTA which in turn is oxidized to TA. TA is subsequently esterified using methanol, with the removal of by-product water, to form DMT. In Route 2, after p-xylene is oxidized to PTA, the PTA is esterified with methanol to form MPT. The MPT is then further oxidized to MMT which in turn is esterified with methanol to form DMT.
In accordance with both of Routes 1 and 2 an intermediate oxidation takes place whereby p-xylene is first oxidized to p-tolualdehyde, which, in turn, is oxidized to p-toluic acid. Thus, the first step of each of Routes 1 and 2 as shown above may also be represented as follows: ##STR2##
In the foregoing process as generally described, an oxidation catalyst is used in the various oxidation steps. While the art frequently mentions the use of "heavy metals" or "heavy metal salts" as oxidation catalysts, in current commercial practice the catalyst is usually cobalt acetate as the sole or major oxidation catalyst component. When a co-catalyst is used, it is usually manganese acetate. These metal salts are water soluble and the total concentration of metal catalyst(s) is generally in the range 0.001-0.1 wt % of the total oxidation reaction mixture. The oxidation catalysts are employed in all oxidation steps in both Routes 1 and 2 described above.
It is recognized that Routes 1 and 2 contain multiple oxidation and esterification steps. This results in considerable capital outlay for multiple reactor sections and in large operating cost outlays for the multiple processing steps. Further, because the rate of oxidation of p-xylene is relatively slow, large steady-state concentrations must be maintained in commercial operations in order to make the process commercially viable. Overall, it would be advantageous if capital and operating costs could be lowered by improvements in the process.
U.S. Pat. No. 4,353,810 (the '810 patent) describes a process for the production of DMT. As described in the '810 patent, the DMT process charges p-xylene, methyl p-toluate (MPT), air (the oxidizing agent) and the oxidation catalyst(s) to a first oxidation stage at a selected oxidation temperature for a selected oxidation time to produce a first oxidation mixture or oxidate. The oxidate is withdrawn from the first stage and passed to a first esterification stage, with or without removal and recycle of unreacted p-xylene. Methanol is admixed with the oxidate and esterification is carried out at a selected temperature for a selected time. Subsequent to esterification, the reaction mixture is distilled to yield MPT (and p-xylene if not previously separated) overhead and a first bottoms fraction. The MPT (and any p-xylene) is returned to the first oxidation stage and the first bottoms fraction is further distilled to yield at least a DMT fraction and a second bottoms fraction. The DMT may be further purified as required and the second bottom fraction may be "discarded" generally by combustion) or recycled, with or without further processing steps. Due to cost and environmental consideration, it is preferred that the second bottoms fraction be recycled or at least the catalyst(s) recovered before the fraction is discarded.
U.S. Pat. No. 5,679,847 to Ohkoshi et al, assigned to Mitsubushi Gas Chemical Co., discloses a process for producing terephthalic acid by oxidizing a liquid-phase starting raw material containing p-xylene having 3 to 35 weight percent p-tolualdehyde based on the same at a temperature in the range of 120.degree. to 240.degree. C. by means of an oxygen-containing gas using a lower aliphatic monocarboxylic acid such as acetic acid, propionic acid or butyric acid as a solvent in the presence of a bromine compound, a manganese compound catalyst and a cobalt compound catalyst. Caution must be exercised during the process to avoid oxidation loss of the solvent monocarboxylic acid.
Japan Open Patent No. 9122311976 (filed Feb. 10, 1976) to Mitsubishi Gas Chemical (MGC1) describes a process for producing terephthalic acid by the liquid phase oxidation of p-tolualdehyde with an oxygen-containing fluid in the presence of a catalyst combination consisting of a manganese compound, a cobalt compound, a bromine compound and a lanthanide(s) metal compound. Additional MGC patents describing the use of p-tolualdehyde include Japan Open Patent Nos. 86433/1976 (hereafter MGC2, "Manufacturing Method of Aromatic Carboxylic Acids and/or Their Methyl Esters"), 86436/1976 (hereafter MGC3, "Manufacturing Method of Toluic Acid from Tolualdehyde"), and 86437/1976 (hereafter MGC4, "Preventing Method of Formation of Black Colored Phthalic Acid"). While the four MGC disclosures discuss the use of p-tolualdehyde in the production of terephthalic acid, they also reveal a serious defect in that the resulting product is frequently black or black-flecked due to the presence of manganese containing substances. Consequently, while the art for the production of TA or DMT is well advanced, further improvements are still desirable. In particular, improvements in product color, reduction of steady-state concentrations, reduced reaction temperature and time, improved reaction rate and/or selectivity, and improved safety are particularly desired.
It is an object of the present invention to describe a process for the production of DMT from p-tolualdehyde, or a reaction mixture containing p-tolualdehyde and additional DMT precursor substances.
It is an object of the present invention to describe a process for the production of DMT from p-tolualdehyde or a p-tolualdehyde-containing reactive mixture whereby the steady-state concentration of feed material may be reduced without loss of overall product output.
It is a further object of the invention to describe a process for the production of DMT having improved safety considerations due to reduced oxidizer oxygen concentrations during startup.
It is an additional object of the invention to describe a process for the production of DMT in which the use of one or more catalysts is reduced or eliminated.